The present invention relates to an improved thin-film electroluminescent device with increased reliability, safety and which may be used in a confined space.
The vast majority of computer displays are designed for placement on a desk or affixed to a portable computer, and accordingly have a rather large viewing area and associated housing for the placement of electronics therein. The construction of such displays, and in particular passive thin film electroluminescent displays (TFEL), have not been primarily concerned with designing the display for use in a much more confined space such as a head-mounted display.
TFEL displays are generally constructed with an electroluminescent phosphor layer sandwiched between dielectric layers, all three layers thereof located between a transparent front electrode layer and a rear electrode layer (all of which is described in greater detail in the detailed description of the preferred embodiment). Referring to FIG. 1, the rear electrode layer of the display 306 has a plurality of individual rear electrodes, each of which is electrically connected to a separate wire or line within an interconnect 300. Likewise, the front electrode layer has a plurality of individual front electrodes, each of which is electrically connected to a separate wire or line within an interconnect 302. Collectively, the wires in the interconnects 300 and 302 extending from the front electrodes and rear electrodes are typically routed with a plurality of elastomer interconnects. An elastomer interconnect is a relatively thin strip of parallel wires enclosed within a plastic casing. The interconnects are routed and electrically connected to a circuit board 304 at a distant location from the display 306. Such interconnects tend to have reliability problems transmitting signals over distances greater than 1/2 inch, due in part to the large number of wires or lines. The circuit board 304 includes row drivers 308 and column drivers 310 to illuminate selected pixels within the display 306. An interface controller 312 includes a frame buffer and timing controller to control the column drivers 310 and row drivers 308 through a set of lines 309 and 311. A data source 314, such as a general purpose computer, provides data representative of the desired image on the display 306 to the interface controller 312. An EL drive 316 provides a high voltage signal that is transmitted to the display 306, generally by the row drivers 308, for the illumination of selected pixels. The transmission of high voltage signals near a person's body, such as from the person's belt to his head, may present safety problems from prolonged exposure. The display 306, interconnects 300 and 302, and circuit board 304 may be enclosed within a single rather bulky housing. However, when such a display is used in an environment with limited available space, such as in a hand-held cellular telephone, head mounted display, or other device, the interconnects 300 and 302, and the circuit board 304 may require more space than is available. Further, locating the display 306, interconnects 300 and 302, and circuit board 304 within a restrictive space may necessitate an awkward and possibly expensive package. For example, an active matrix TFEL display with 1280 row electrodes and 1024 column electrodes would require over 2000 lines within an interconnect or plurality of interconnects, either of which requires substantial space and expense. Additionally, such a large number of lines decreases the reliability of transmitting signals.
Active matrix thin film electroluminescent displays (AMEL display or device) are frequently rather small, such as 1.3".times.1.2". AMEL displays could be used in a wide range of applications, such as head mounted displays. The front of a head mounted display typically includes two separate displays, each of which is mounted on the headset for viewing by a respective eye of the wearer. An AMEL device typically includes a transparent electrode, a rear circuit layer that includes both row and column electrodes, and an interdisposed sandwich structure of a dielectric, phosphor, and dielectric (all of which is described in greater detail in the detailed description of the preferred embodiment). The main functional differences between an AMEL and a passive TFEL display are that each pixel in an AMEL display has its own switch to turn it on and off, and pixels in an AMEL display could be on nearly 100% of the time while pixels are on only a fraction of the time in passive TFEL displays. If an AMEL display wire is interconnected to the row drivers, column drivers, and other electrodes in a manner similar to passive TFEL devices, a separate wire would be required for each of the row and column electrodes in the rear circuit layer. The wires would have to be routed in the form of one or more elastomer interconnects to a circuit board. Accordingly, an AMEL display that has 1280 row electrodes and 1024 column electrodes would require over 2000 wires to transmit signals from the circuit board to the display, preferably in one or more elastomer interconnects. The circuit board would be located on a wearer's belt in a head-mounted display application. The physical size of the interconnects, presents a problem when routed from a wearer's head to his belt. The major problem is that the interconnects are large, bulky and heavy. Bulky and heavy head-mounted displays are less desirable and less comfortable for users. Interconnects with a large number of wires may be difficult to route from the front of the headset to the remotely located circuit board, add unnecessary expense, decrease the reliability of signal transmission, add unnecessary weight, and are prone to damage.
Referring to FIG. 2, an AMEL display is fabricated on a substrate 320, typically made of silicon. By providing a substrate 320 that is slightly larger than the display area 328, row drivers 322 and column drivers 324 may be fabricated on the substrate 320. Locating the row and column drivers 322 and 324 on the substrate 320 eliminates the need for the row and column drivers to be located on the circuit board 330. A plurality of bus lines 332 are fabricated on the outer portion of the substrate 320 to transmit signals from one location of the display to another. The display, as shown in FIG. 2, reduces the number of lines or wires required from the interface controller 326 to the display to approximately 100 lines or wires, depending upon the number of pixels in the device. Although significantly reduced from thousands, one hundred wires still requires a large interconnect. Transmitting high voltage and high frequency signals in the bus lines 332 near the display area 328 causes interference to the electronics in the row drivers 322 and column drivers 324. Also, the bus lines 332 require a substantial amount of substrate area to fabricate. Locating the bus lines 332 further from the row and column drivers 322 and 324, respectively, requires more substrate area, which in turn increases the size and expense of the display. In an attempt to decrease the amount of substrate 320 required the bus lines 332 may be made thin and placed closer together but this reduces the conductivity of the bus lines 328 and creates significant coupling between bus lines 332. Further, the choice of metals that can be deposited on a silicon substrate is limited, and those metals that are suitable have less conductivity than more desirable conductors such as gold and copper. In an attempt to compensate for the less conductive metals, wider lines are necessary to obtain desirable conductivity. However, wider lines require more substrate area.
In addition, a safety concern arises from the transmission of high voltage signals near the body of the wearer from an EL drive to the row drivers in a passive TFEL display and to the transparent electrode layer in an AMEL display. If the insulator on the wire is cut, or otherwise damaged, then the wearer could be receive a high voltage shock. Additionally, electromagnetic interference (EMI) from the high voltage wire may cause erroneous data in nearby data lines.
What would be desirable, therefore, is a display that eliminates the aforementioned drawbacks associated with the interconnects and permits the display, including the driving circuitry, to be packaged within a more limited space. Additionally, the size of the substrate should be minimized to reduce the cost of the device while allowing a greater diversity of more conductive metals to be selected for the bus lines. The high voltage lines routed to the display should be eliminated to alleviate safety concerns.